Why is it only okay to slurp noodles in Japan?

The most common explanation is that slurping makes noodles taste better because you can eat them hotter, the oyaji (older men) who slurp their soup, coffee and rice would probably say the same about those things. Slurping noodles also seems to help with no getting whiplash and hence stains all over your shirt (or even your neighbour), so I’d suggest that this is more of a reason than the taste thing.